3.Human Physiology

3.1 .Circulation

Circulation

Dr V Malathi

The Circulatory System

The circulatory system is also known as the cardiovascular system.

The chief function of the circulatory system includes transporting blood, nutrients, oxygen, carbon dioxide, and hormones throughout the body. It consists of the heart, blood vessels (arteries, veins, and capillaries), and blood.

Key Components of the circulatory system 

  1. Heart: A muscular organ that pumps blood throughout the body.
  2. Arteries: Blood vessels that carry oxygenated blood away from the heart.
  3. Veins: Blood vessels that carry deoxygenated blood back to the heart.
  4. Capillaries: These tiny blood vessels permit the exchange of oxygen, carbon dioxide, and other substances between blood and tissues.

Major Functions of the circulatory system:

  • Oxygen and Nutrient Transport: Delivers oxygen and nutrients to cells.
  • Waste Removal: Removes carbon dioxide and other metabolic wastes from cells.
  • Hormone Distribution: Transports hormones to target organs and tissues.
  • Temperature Regulation: Helps maintain body temperature by distributing heat.

File:Circulation of blood through the heart.png

“Circulation of blood through the heart” by Christinelmiller via Creative commons .org Via Wikimedia commons is licensed under CC BY-SA 4.0

The Heart

Video Links

Watch the on Human Heart Anatomy (3D Medical Animation) from Arcreative Media

 

  • The heart is a muscular organ that pumps blood through the three divisions of the circulatory system namely
    • the coronary (vessels that serve the heart),
    • pulmonary (heart and lungs), and
    • systemic (systems of the body)

Structure of the Heart 

The myocardial, endocardium, and epicardium are the three layers that make up the heart.

The endocardium is the lining that lines the inside of the heart.

The middle layer of the heart wall and the majority of its heart muscle cells make up the myocardium.

The pericardium, a layered structure made of membranes that surrounds and protects the heart, is the second layer of cells that make up the epicardium.

This layer gives the heart enough room for vigorous pumping while also keeping it stationary to lessen friction between the heart and other structures.

The heart contains four chambers namely:

  • Right Atrium: Receives deoxygenated blood from the body.

The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself.

  • Right Ventricle: Pumps deoxygenated blood to the lungs.

The deoxygenated blood from heart then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The biscuspid or mitral valve separates the chambers on the left side of the heart.

 After the right ventricle is filled, it pumps the blood through the pulmonary arteries. This blood passes through the semilunar valve (or pulmonic valve) and is taken to the lungs for re-oxygenation.

After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle.

  • Left Atrium: Receives oxygenated blood from the lungs.
  •  The left atrium then receives the oxygen-rich blood from the lungs through the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle

Left Ventricle: Pumps oxygenated blood to the body.

From the left ventricle the blood is pumped out through aorta.

The aorta is the major artery of the body, taking oxygenated blood to the organs and muscles of the body.

Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing backward flow of blood into the left ventricle.

This pattern of pumping is referred to as double circulation and is found in all mammals.

The pumping action of the heart is a function of the cardiac muscle cells, or cardiomyocytes,

These cardiomyocytes make up the heart muscle.

Cardiomyocytes, are striated muscle cells. They pump rhythmically and involuntarily. They are connected by intercalated disks exclusive to cardiac muscle.

 

The Cardiac Cycle

The repeating sequence of pumping of blood by the heart is called Cardiac cycle ,

Electrical signals cause the heart muscles to contract and relax. This causes the filling and emptying of the heart with blood

The human heart beats over 100,000 times per day.

In each cardiac cycle, the heart contracts ( systole), pushing out the blood and pumping it through the body;

This is followed by a relaxation phase ( diastole), where the heart fills with blood.

The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles.

The closing of the atrioventricular valves produces a “lup” sound.

Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery).

Closing of the semilunar valves produces a “dup” sound.

Blood Vessels

  1. There are three types of blood vessels: the arteries, veins, and capillaries.
  2. Arteries 

Arteries are blood vessels that carry oxygenated blood away from the heart to various parts of the body.

Layers of Artery Walls include : 

There are three distinct layers, or tunics, that form the walls of blood vessels; namely

    • Tunica Intima: The innermost layer, consisting of endothelial cells and elastic fibers.
    • Tunica Media: The middle layer, made up of smooth muscle cells and elastic fibers, which helps control the diameter of the artery.
    • Tunica Externa: The outermost layer, composed of elastic fibers and collagen, providing structure and support.

Types of Arteries:

      • Elastic Arteries: These are closer to the heart (e.g., the aorta) and contain more elastic fibers to handle the high pressure of blood pumped from the heart.
      • Muscular Arteries: These are further from the heart and contain more smooth muscle tissue, allowing them to regulate blood flow by constricting or dilating.

Major Arteries

  1. Aorta: The largest artery in the body, originating from the left ventricle of the heart and branching into smaller arteries.
  2. Coronary Arteries: Supply blood to the heart muscle itself.
  3. Carotid Arteries: Supply blood to the brain, neck, and face.
  4. Femoral Arteries: Supply blood to the lower limbs.

Functions of the arteries 

  • Oxygen and Nutrient Transport: Arteries carry oxygenated blood and nutrients to tissues and organs.
  • Blood Pressure Regulation: The smooth muscle in the tunica media helps regulate blood pressure by adjusting the diameter of the arteries.
  • Pulse: The expansion and contraction of elastic arteries with each heartbeat create the pulse that can be felt in various parts of the body.

Arterial Circuits:

  • Systemic Circuit: Carries oxygenated blood from the heart to the body.
  • Pulmonary Circuit: Carries deoxygenated blood from the heart to the lungs for oxygenation

Veins

  1. Layers of Vein Walls:
    • Tunica Intima: The innermost layer, consisting of endothelial cells.
    • Tunica Media: The middle layer, thinner than in arteries, containing smooth muscle cells and elastic fibers.
    • Tunica Externa: The outermost layer, composed of connective tissue, providing support and structure.

Types of Veins:

      • Superficial Veins: Located close to the surface of the skin.
      • Deep Veins: Located deeper within the body, usually alongside arteries.
      • Pulmonary Veins: Carry oxygen-rich blood from the lungs to the heart.
      • Systemic Veins: Carry deoxygenated blood from the body back to the heart.

Functions of Veins 

  • Blood Return: Veins return deoxygenated blood from the body to the heart, except for the pulmonary veins, which carry oxygenated blood from the lungs.
  • Valves: Veins contain one-way valves that prevent the backflow of blood, ensuring it moves towards the heart.
  • Blood Reservoir : Veins act as a reservoir and hold about 75% of the body’s blood

Major Veins 

  1. Superior and Inferior Vena Cava: Large veins that return deoxygenated blood from the body to the right atrium of the heart.
  2. Jugular Veins: Drain blood from the head and neck.
  3. Femoral Veins: Drain blood from the lower limbs.

Venous Circuits:

  • Systemic Circuit: Returns deoxygenated blood from the body to the heart.
  • Pulmonary Circuit: Returns oxygenated blood from the lungs to the heart.

Capillaries

Capillaries are the smallest and most numerous blood vessels in the body, playing a crucial role in the circulatory system

Structure:

The capillaries are tiny vessels with a diameter of about 5 to 10 microns

Layers:

    • Endothelial Cells: The inner lining of capillaries, which controls the exchange of substances between the blood and surrounding tissues.
    • Basement Membrane: A thin, supportive layer surrounding the endothelial cells.

Types of Capillaries:

  • Continuous Capillaries: Have a continuous endothelial lining and are found in muscles, skin, and the central nervous system. They allow the passage of small molecules like water and ions.
  • Fenestrated Capillaries: Have pores (fenestrae) in their endothelial lining, allowing for the rapid exchange of substances. These are found in the kidneys, intestines, and endocrine glands.
  • Sinusoidal Capillaries: Have larger openings and a discontinuous basement membrane, allowing the passage of larger molecules and cells. These are found in the liver, spleen and bone marrow.

Functions of Capillaries 

  • Exchange of Gases: Capillaries facilitate the exchange of oxygen and carbon dioxide between blood and tissues.
  • Nutrient and Waste Exchange: They allow nutrients to pass from the blood to tissues and waste products to move from tissues to the blood.
  • Hormone Delivery: Capillaries transport hormones to target organs and tissues.
  • Fluid Balance: They help maintain fluid balance by allowing the movement of water and solutes between blood and tissues.

Capillary Beds:

  • Network: Capillaries form extensive networks called capillary beds, which increase the surface area for exchange.
  • Regulation : Blood flow through capillary is regulated by precapillary sphincters

For further learning about circulatory system visit the pressbook on Circulatory system from BC campus 

Watch the video from drugs.com to know how a healthy heart works?

 

 

Test your Understanding about the Circulatory system by attempting the interactive exercises below

 

 

 

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3.1 .Circulation by Dr V Malathi is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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